1. Field of the Invention
This invention relates generally to the field of rigid disc drive data storage devices and more particularly, but not by way of limitation, to a magnetic latch for the carriage that mounts and carries the heads in such a disc drive.
2. Brief Description of the Prior Art
Disc drive data storage devices of the type known as "Winchester" disc drives are well known in the industry. In such units, a plurality of rigid discs, coated with a magnetic recording medium, are mounted for rotation at a constant high velocity on a spindle motor. An actuator mechanism is included to mount a plurality of heads--typically one per disc surface--and move the heads under control of electronic circuitry to any desired one of a plurality of circular, concentric data tracks on the discs. The actuator mechanism commonly includes some sort of precision guidance apparatus which closely controls the path of movement of the heads, and a motor to drive the heads through their range of motion. That portion of the actuator that mounts the heads is commonly referred to as a "carriage".
Most disc drives of the current technology utilize a type of actuator known as a rotary voice coil actuator. In such a unit, the heads are typically attached via flexures to a vertically aligned array of head mounting arms which are cantilevered outward from an actuator body which is adapted to rotate about a pivot shaft. On the opposite side of the actuator body from the head mounting arms, a coil is fixedly attached, and this coil is supported by the actuator body within the magnetic flux field of an array of permanent magnets. When controlled DC current is applied to the ends of the coil, the magnetic field induced about the coil interacts with the magnetic field of the permanent magnets to rotate the coil about the pivot shah in accordance with the well known Lorenz relationship.
In disc drives of the current technology, such motors have enabled manufacturers to produce products with average access times of less than ten milliseconds (0.010 seconds). As is well known to those skilled in the art, it is necessary to "park and latch" such actuators when the loss of power to the disc drive unit is detected, because voice coil motors have no inherent "magnetic detent" and can only control the relative position of the coil and magnets when power is available to the coil. When a power loss occurs, the discs in a disc drive stop spinning, and the air bearing which supports the heads above the disc surface begins to deteriorate. Since the heads and discs are relatively delicate, it is imperative that uncontrolled contact between the two be eliminated. This is typically done by rapidly moving the heads to a preselected "parking zone" and latching the heads in this position until the restoration of power is detected.
With a voice coil actuator, it is relatively simple to move the heads to a park area, since the application of a fixed DC current of the proper polarity to the coil of the motor will cause the desired motion. Several different schemes have been used to latch the carriage of a disc drive at the park area, including spring-activated latches, solenoid latches, and magnetic latches. A typical spring activated carriage latch includes a spring-biased latch pawl which engages a feature on the carriage when the carriage is moved to the park position. Such a latch requires that some device be included to overcome the spring bias in order to unlatch. This can take any of several forms, including wind vanes which move in response to the acceleration of the discs when power is restored to the disc drive.
Another type of unlatching mechanism frequently used is a solenoid, which must be energized at the proper moment in the power-up sequence for the disc drive to open, or unlatch, the latch. Several types of latches incorporating bi-stable solenoids have been proposed including those described in U.S. patent applications No. 711,311, filed Jun. 6, 1991, now U.S. Pat. No. 5,224,000, issued Feb. 16, 1993, and U.S. patent application No. 866,611, filed Apr. 10, 1992, (Attorney's Docket SEA 8154, Sampietro, et al.), both assigned to the assignee of the present invention and incorporated herein by reference. The main drawback of such latches is that they require that power be used to both latch and unlatch the mechanisms, and power to latch the mechanism at the detection of power loss can be unreliable.
Magnetic carriage latches are also common in the art, such as U.S. Pat. No. 4,725,907, issued Feb. 16, 1988, and U.S. patent applications No. 611,412, filed Nov. 9, 1990, now U.S. Pat. No. 5,224,000, issued Feb. 16, 1993 and U.S. patent application No. 894,417, filed Jun. 5, 1992, (Attorney's Docket SEA 2206, Jue, et al.), all also assigned to the assignee and incorporated herein by reference. These are surely the simplest latches in concept, since they usually incorporate a permanent magnet mounted to a fixed position in the disc drive housing that contacts a striker surface on the actuator when the actuator has been moved to the designated park position. Power to overcome the magnetic latch, and thus release the actuator for movement, is provided by the actuator motor itself. The principal obstacle to be overcome with a magnetic latch is a compromise between the available latching force and the amount of power necessary to overcome the latching force at power-on.
That is, if the latch is capable of exerting a large amount of latching force to hold the carriage in the park position against large amounts of specified non-operating shock, the amount of power required by the actuator motor to overcome the latching force at power on goes up dramatically.
It would obviously be advantageous to have a simple magnetic latch which provides a large amount of latching power, while not requiring a comparably large amount of force be prodded by the actuator motor to unlatch the carriage.